The combo of titanium line and hi-vis extender is pretty clearly intended for stream fishing.

It is only a fast sinking line if you don't hold it off the water's surface. The titanium line is much denser than fluorocarbon, but that allows you to use a much thinner line (which creates a virtuous circle, allowing you to overcome wind resistance more easily). The titanium line I have has the diameter of 3X tippet material. It has sufficient weight to cast but should not be so heavy that you can't keep it off the water.

Watch some of the videos on tenkara.com. He uses titanium line and is able to keep his line in bank eddies on the other side of current tongues for a truly surprisingly long time - which would not be possible if his line was sinking.

For still water anglers fishing longer lines, I do not see a point to using a hi-vis extender. You can't keep a long line off still water, and a titanium line is not going to float. As you say, a hi-vis extender is no longer hi vis when it is under water 20' away.

If you wanted to fish deep in still water, and you had a rod that was stiff enough to pull the line back out of the water to make your next cast, it might work very well. Other than that, I don't see an application for titanium lines in still water.